This invention relates to improvements in motor circuits for use in electric power assisted steering systems, and in particular to a method for controlling the motor during a known fault mode in which the motor may be unable to produce an output torque at certain rotor conditions.
It is known to provide a motor in an electric power assisted steering system which is configured in such a way that a torque output from the motor will help the driver of the vehicle to turn the steering wheel. The motor is typically connected to a steering column or rack through a gearbox, and a controller demands a torque from the motor which has a value that is a function of the torque applied to the steering wheel by a driver. The demanded torque is converted into drive currents that are applied to the motor.
Motor drive circuits for such motors using feedback control and PWM are well known in the art. For example, WO2006005927, discloses a typical system and the teaching of that document is incorporated herein by reference.
During steady state operation of the motor, where the controller is demanding a constant torque and rotating at a constant speed, the voltages applied to each of the phases of a three phase motor may be chosen so that the current in each phase varies sinusoidally over an electrical revolution of the motor rotor, the frequency of the signal being chosen according to the speed of rotation of the rotor of the motor, and the magnitude of the signals being dependent on the required torque. By arranging for each of the current waveforms to be offset from the others by 120 degrees the overall current carried by the motor will be constant and so the motor torque will also be constant as the motor rotates. Applying currents in this pattern ensures that the motor rotates smoothly with no peaks in torque. This is well known in the art and the theory behind this will therefore not be described in detail here.
Most motors used in electric steering systems are three phase, as described above. It is also known to provide a fault mode of operation for the motor to allow the motor to continue providing some assistance torque in the event that one of the phases of the motor fails in an open circuit leaving only two phases able to conduct current. The controller may continue as in the normal mode of operation by applying the sinusoidal waveforms that would be used if all three phases are working. The torque of the motor will vary as the motor rotates. However there will be two points which are referred to here as zero torque points where the rotor is perfectly aligned with the magnetic field and so cannot generate any torque. When the motor is used in an electric power steering, these points of zero torque will be felt through the steering wheel as a torque ripple which may worry a driver.
In many instances, although annoying to the driver, the presence of the zero torque points do not significantly impair the ability of the driver to turn the wheel. However, if the steering wheel is being turned at low speeds with a high demanded torque, the disappearance of the torque at the zero point will produce a very significant steering feel anomaly and in certain cases could leave the driver unable to turn the steering wheel. This gives the driver the feeling of the steering being pushed into a “notch” which the driver can find hard to push past.